In the past, the MRI phenomenon has been utilized by structural chemists to study, in vitro, the molecular structure of organic molecules. Typically, MRI spectrometers utilized for this purpose were designed to accommodate relatively small samples of the substance to be studied. More recently, however, MRI has been developed into an imaging modality utilized to obtain images of anatomical features of live human subjects. Such images depicting parameters associated with nuclear spins (typically hydrogen protons associated with water in tissue) may be of medical diagnostic value in determining the state of health of tissue in the region examined. The use of MRI to produce images and spectroscopic studies of the human body has necessitated the use of specifically designed system components, such as the magnet, gradient and RF coils.
In imaging techniques using the MRI phenomenon, it is necessary that the subject to be imaged remains motionless. Because known imaging techniques span time periods of typical heart and respiratory cycles, some movement of the subject is inevitable. A known method of avoiding distortion of an MR image from biological motion such as heart and lung movement, is to gate the acquisition of MRI signals to the cyclic movement of the heart or lungs. Unfortunately, in order to gate the acquisition of MRI signals to body movement such as heart or lung motion, it has been necessary to place probes on or in close proximity to the subject. This requirement results in probes being placed inside the bore of the main magnet--an undesirable situation since the probes may often generate distortions in the uniform magnetic field B.sub.0 and/or in the radio frequency field, B.sub.1, with a resulting reduction in image quality. The necessity of applying probes to the patient also reduces scanner throughput thereby increasing the cost per scan.